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<h4 class="subsection">5.1.17 <code>CONVERT</code> specifier</h4>
<a name="index-CONVERT-specifier"></a>

<p>GNU Fortran allows the conversion of unformatted data between little-
and big-endian representation to facilitate moving of data
between different systems.  The conversion can be indicated with
the <code>CONVERT</code> specifier on the <code>OPEN</code> statement.
See <a href="GFORTRAN_005fCONVERT_005fUNIT.html#GFORTRAN_005fCONVERT_005fUNIT">GFORTRAN_CONVERT_UNIT</a>, for an alternative way of specifying
the data format via an environment variable.
</p>
<p>Valid values for <code>CONVERT</code> on most systems are:
</p><ul class="no-bullet">
<li><!-- /@w --> <code>CONVERT='NATIVE'</code> Use the native format.  This is the default.
</li><li><!-- /@w --> <code>CONVERT='SWAP'</code> Swap between little- and big-endian.
</li><li><!-- /@w --> <code>CONVERT='LITTLE_ENDIAN'</code> Use the little-endian representation
for unformatted files.
</li><li><!-- /@w --> <code>CONVERT='BIG_ENDIAN'</code> Use the big-endian representation for
unformatted files.
</li></ul>
<p>On POWER systems which support <samp>-mabi=ieeelongdouble</samp>,
there are additional options, which can be combined with the others
with commas. Those are
</p><ul class="no-bullet">
<li><!-- /@w --> <code>CONVERT='R16_IEEE'</code> Use IEEE 128-bit format for
<code>REAL(KIND=16)</code>.
</li><li><!-- /@w --> <code>CONVERT='R16_IBM'</code> Use IBM <code>long double</code> format for
real<code>REAL(KIND=16)</code>.
</li></ul>

<p>Using the option could look like this:
</p><div class="smallexample">
<pre class="smallexample">  open(file='big.dat',form='unformatted',access='sequential', &amp;
       convert='big_endian')
</pre></div>

<p>The value of the conversion can be queried by using
<code>INQUIRE(CONVERT=ch)</code>.  The values returned are
<code>'BIG_ENDIAN'</code> and <code>'LITTLE_ENDIAN'</code>.
</p>
<p><code>CONVERT</code> works between big- and little-endian for
<code>INTEGER</code> values of all supported kinds and for <code>REAL</code>
on IEEE systems of kinds 4 and 8.  Conversion between different
&ldquo;extended double&rdquo; types on different architectures such as
m68k and x86_64, which GNU Fortran
supports as <code>REAL(KIND=10)</code> and <code>REAL(KIND=16)</code>, will
probably not work.
</p>
<p><em>Note that the values specified via the GFORTRAN_CONVERT_UNIT
environment variable will override the CONVERT specifier in the
open statement</em>.  This is to give control over data formats to
users who do not have the source code of their program available.
</p>
<p>Using anything but the native representation for unformatted data
carries a significant speed overhead.  If speed in this area matters
to you, it is best if you use this only for data that needs to be
portable.
</p>
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